Early reports from event organizers are hailing the turnout as the largest climate march in history, far bigger than the Forward on Climate rally held in Washington, D.C., last year. High-profile environmentalists including Bill McKibben, Leonardo DiCaprio, Jane Goodall and Vandana Shiva marched alongside policymakers such as Sens. Sheldon Whitehouse (D-R.I.), Bernie Sanders (I-Vt.) and Charles Schumer (D-N.Y.). U.N. Secretary-General Ban Ki-moon and former Vice President Al Gore were also there, and more than 550 buses carried in people from around the country.

The rally comes at an opportune time as 120 world leaders, including President Barack Obama, are expected to convene Tuesday at the United Nations in New York to discuss ways to tackle the growing threat of carbon pollution.

The march began around 11:30 a.m., at New York City's Columbus Circle just off Central Park. At times, it stretched more than 4 miles as marchers carried banners, signs and entire contraptions depicting everything from Mother Earth herself to the dinosaurs that now make up fossil fuels.

Demonstrators make their way down Sixth Avenue in New York during the People's Climate March on Sunday.

"Today I march because I want to behold a brighter future. We have destroyed ourselves. We have destroyed our health and I'm here because our political leaders have failed us," Stanley Sturgill, a retired coal miner from Kentucky now suffering from black lung, said at a press conference before the march. "We know together we can build our bright future."

More than 1,500 groups filled Central Park West before the march. They represented a variety of interests, including the scientific community and religious organizations. More than 50,000 students were there because they were worried about their future, while grandparents came out of concern for their legacy.

"We need to act now ... We only have one atmosphere and we of the Marshall Islands only have one land to call 'home,'" Kathy Jetnil-Kijiner, a young mother from the island nation, said before the march. "We don't want to move and we shouldn't have to move."